1. Field of the Invention
The present invention relates to a differential gear for transmitting torque from one input shaft to two output shafts and effecting differentiation between the two output shafts with different rotational speeds.
2. Description of the Prior Art
In a vehicle such as an automobile having a pair of right and left driving wheels, a differential gear is used for the purpose of transmitting engine power to both the driving wheels. In turning the vehicle, there is a difference in the rolling distance between the right and left driving wheels due to a difference in the turning radius. This difference causes the slip of one of the driving wheels. To prevent this slip, the differential gear effects differentiation between the driving wheels.
Generally, the differential gear employs a differential mechanism comprising a differential casing, a differential pinion gear and a side gear. In this differential gear, the same torque is always transmitted to both the driving wheels. Therefore, when one of the driving wheels sticks in the mud, snow or icy road, the torque transmitted to the other driving wheel is decreased, causing a reduction in operating performance. To overcome this problem, a limited slip differential has been proposed for generating an effective friction torque in the differential gear and increasing the torque to be transmitted to a driving wheel when rotating at lower speeds.
A conventional limited slip differential includes a torque proportional type limited slip differential having a cam mechanism on a differential pinion shaft portion for generating a thrust force and a multiple disc clutch friction mechanism for generating a differential resistance upon application of the thrust force. Japanese Patent Laid-open No. 51-89068 discloses a speed responsive differential mechanism having a friction clutch adapted to be engaged in response to a difference in speed between a differential casing and a side gear.
The torque proportional type limited slip differential is designed to limit differentiation by utilizing a thrust force generated by a transmitted torque. Accordingly, when torque is applied during a sharp turn at low speeds, the differentiation is limited and slip is created. Further, it is desirable to limit the differentiation between both driving wheels during straight movement at high speeds, to ensure stability in running. However, since the torque is small during high-speed running, the limitation of differentiation is not effected. When one of the driving wheels is in a complete slipping condition, torque is not transmitted, and the limitation of differentiation is not effected. Accordingly, it is impossible to transmit torque to the other driving wheel and get out of the slipping condition.
While the speed responsive differential mechanism disclosed in Japanese Patent Laid-open No. 51-89068 is responsive to speed rather than torque, it is responsive to a difference in speed between the differential casing and the side gear, but not responsive to a vehicle speed. Therefore, it is impossible to limit the differentiation between both driving wheels during straight movement at high speeds and thereby improve the running stability.